1. Field of the Invention
The present invention relates to a display device, and in particular to a digital display device having a display element such as an emissive element in each pixel and in which the elements are operated using digital signals to represent gradations.
2. Description of the Related Art
Because electroluminescence (hereinafter simply referred to as “EL”) display devices in which a self-emitting EL element, for example, is used as an emissive element in each pixel have advantages such as that the device is thin, self-emitting, and consumes less power, EL display devices have attracted much attention as alternatives to display devices such as liquid crystal display (LCD) and cathode ray tube (CRT) display devices.
In particular, a high resolution display can be achieved by an active matrix EL display device in which a switching element such as a thin film transistor (hereinafter simply referred to as “TFT”) for individually controlling an EL element is provided in each pixel and the EL element in each pixel is controlled.
In an active matrix EL display device, a plurality of pixels, a plurality of selection lines (gate lines) extending along a horizontal scan direction (row direction), a plurality of data lines and power supply lines extending along a vertical scan direction (column direction) are provided over a substrate, and each pixel has an organic EL element, a selection TFT, a driver TFT, and a storage capacitor. In this structure, a selection signal is output to a selection line so that each of the selection TFTs connected to this line is switched on, a data signal (analog voltage signal) output onto the data line is supplied to the storage capacitor and to the driver TFT, the storage capacitor stores a voltage corresponding to the data signal for a predetermined period, and the driver TFT is operated to control electric current to be supplied from the power supply line through the organic EL element.
In addition to a method for driving each organic EL element with an analog data signal, a method is reported in which each organic EL element is driven with a digital data signal as shown in FIG. 1 (digital drive). In a pixel circuit shown in FIG. 1, a TFT 26 for switching current on and off is additionally provided, in a circuit structure for driving an EL element with an analog signal as already described, between an organic EL element 28 and a driver TFT 22 which is connected between an EL power supply and the organic EL element 28 for controlling supply of current to the organic EL element 28. When a selection signal is output to the gate line and the selection TFT 20 is switched on, a digital signal output onto the data line is supplied to and stored in the storage capacitor 24 through the selection TFT 20 and also is applied to a gate of the driver TFT 22.
The driver TFT 22 is switched on and off according to the digital data signal applied to its gate and the current on-off TFT 26 controls whether or not to supply the current flowing through the driver TFT 22 to the organic EL element 28 to allow the organic EL element 28 to emit light. The current on-off TFT 26 is controlled to be switched on and off a plurality of times during one frame period (one screen display period) through time divisional control corresponding to a number of bits in the digital data, to thereby control the total cumulative light emission period during one frame period for the organic EL element 28. Because light emission intensity as recognized by a viewer differs depending on the length of light emission period within one frame period, with the time divisional light emission control as described, it is possible to represent gradations. In other words, light emission gradation can be represented by merely controlling the length of light emission period of an organic EL element 28 during one frame period.
When a pixel circuit as shown in FIG. 1 is used and gradation is represented by a time divisional digital gradation drive method, in order to achieve a gradation display, analog control of the amount of current to be supplied to the organic EL element 28 is unnecessary, as the driver TFT 22 can be digitally operated to be switched on and off to supply or not supply current to the organic EL element 28. As such, when a current is to be supplied from the driver TFT 22 to the organic EL element 28, by setting a voltage of a data signal so that a large voltage which allows the on resistance of the driver TFT 22 to be sufficiently reduced is applied to the gate of the driver TFT 22, it is possible to reduce influences, to the light emission intensity of the organic EL elements 28 in the pixels, of variations in characteristics among TFTs. Thus, with a digital display method, it is possible to easily inhibit variation in display brightness, that is, display unevenness, among pixels.
In a circuit structure as shown in FIG. 1, however, the operations of switching on and switching off the driver TFT 22 must be directly controlled with a data signal to be applied to the gate of the TFT 22. Therefore, although the data signal is a digital signal, it is necessary to use a digital signal of a large amplitude for sufficiently securing on-off resistance ratio of the driver TFT 22 and to supply the data signal to the gate of the driver TFT 22.
In a matrix type display device, a plurality of pixels each having a circuit structure as shown in FIG. 1 are formed in a matrix, with a data line connected to pixels arranged along a column direction among the plurality of pixels, and a data signal as described above is supplied to the pixels through the data line. In other words, a plurality of pixels arranged along the column direction are connected to each data line, and, from the point of view of the data signal to be applied to each data line, these connected pixels are equivalent to very large parasitic capacitances (capacitance loads) connected to the data line in parallel. Therefore, in order to supply, to a data line to which such large capacitance loads are connected, a data signal having a sufficiently large amplitude to be able to sufficiently control the switching on and off of the driver TFT 22 in each pixel, it is necessary to employ a circuit with a high driving capability.
In addition, in the time divisional digital gradation driving method it is necessary to provide subfield periods each determined by dividing one frame period by a number equal to a number of data bits determined corresponding to the number of display gradations and to output a data signal in each subfield period. Therefore, compared to a method for achieving a gradation display with an analog signal, for example, the transmission speed of the data signal must be increased and a higher and higher transmission speed would be required as the number of display gradations is increased. However, as described above, the parasitic capacitance connected to the data line to which the data signal is to be output is large, and, thus, it is difficult to output a data signal having a large amplitude to allow sufficient control to switch on and off the driver transistor 22 with a sufficiently high speed, to the data line to which a large parasitic capacitance is connected. Therefore, it is not possible to drive the data line with a high speed in order to increase the number of display gradations and the number of gradations that can be displayed is limited.
The present invention therefore provides a digital light emitting device or display device in which a simple driver circuit is employed and to a digital light emitting device or display device which can be driven at a high speed and in which gradation display can be easily achieved.